In a multiple cylinder type internal combustion engine, which has a plurality of cylinders, the use of a so-called multi-point injection (MPI) system, in which an injector for fuel injection is arranged for each cylinder, is generally know. An MPI system provides a high degree of freedom of air intake system, and easily achieves a high output. For this reason, the MPI system attracts attention as a main system for electronic controlled fuel injection.
An MPI system uses a group injection method in which a plurality of cylinders are grouped in advance and the injectors for each cylinder group are operated simultaneously to inject fuel, or a sequential injection method in which a plurality of injectors are operated independently to sequentially inject fuel into each cylinder. Whichever fuel injection method may be used, it is desirable to set the fuel injection timing so as to avoid a stroke having the possibility of deteriorated combustion and deteriorated exhaust gas, and, more particularly, the intake stroke.
In order to set the fuel injection timing for each cylinder or each cylinder group so as to avoid the above mentioned problem of the intake stroke, it is important to determine in which stroke of the combustion cycle each cylinder of internal combustion engine is in. Specifically, a combustion cycle consisting of four strokes of intake, compression, combustion (expansion), and exhaust is repeated in each cylinder of internal combustion engine. For each cylinder, moreover, the timing is set in advance to enter the combustion stroke sequentially at equal intervals. Therefore, if it can be determined in which stroke the particular cylinder is, or inversely which cylinder is in the particular stroke, it can be known in which stroke each of the remaining cylinders is.
The fuel injection described above before is controlled on the basis of such a cylinder discrimination result. At the start of the internal combustion engine, there scarcely arises a problem even if fuel is simultaneously injected into plural cylinders, so that, generally, it is necessary only that the cylinder discrimination is made after cranking is completed.
However, the requirement for cylinder discrimination necessary to control the ignition system is very severe as compared with the cylinder discrimination necessary to control the fuel injection. Incidentally, a high voltage distribution system, which ignites cylinders in sequence by distributor, presents no problem because the cylinder operated by ignition is automatically selected by the distributor. For a low voltage distribution system, which does not use a distributor, it is necessary to make cylinder discrimination as quickly as possible at the engine start so as to determine which ignition coil (cylinder) should be energized.
Conventionally, in order to control the ignition and fuel injection timing for each cylinder, and further to detect the rotational speed, a sensor is mounted on the rotating output shaft (crankshaft) of the internal combustion engine to detect the crank angle. Since the crankshaft rotates two turns in one combustion cycle, however, cylinder cannot be identified directly from the output of the crank angle sensor. A cylinder group consisting of two cylinders with a 360.degree. mutually different stroke phase can be identified from the output of the crank angle sensor, though. Conventionally, therefore, a sensor is also mounted on a camshaft rotating in connection with the crankshaft to determine a 360.degree. difference in stroke phase. Thus, the cylinder can be discriminated by using the signal from the cam sensor and that from the crank angle sensor. The camshaft, which opens and closes the intake and exhaust valves for each cylinder in the valve train, rotates one turn in synchronization with two turns of the crankshaft.
However, if the cylinder discrimination is implemented by two signal systems consisting of the crank angle sensor and the cam sensor, the construction is generally becomes complicated and the cost is increased. Moreover, variations in phase inevitably occur between the signals obtained from the sensors due to the extension/contraction, deflection, etc. of a timing belt for connecting the crankshaft with the camshaft. For this reason, there are possibilities of a cylinder discrimination timing lag and mistaken discrimination.
Unexamined Japanese Patent Publication No. 6-213052 discloses a special sensor that is mounted on the crankshaft to generate a preset reference angle signal and a rotating angle signal. According to this Publication, a method is disclosed in which based on the signal obtained from this sensor, a control signal for each crank angle of 360.degree. with the detection timing of the reference angle signal being the reference is obtained, and the group injection of fuel and group ignition for the plural cylinders are effected according to the control signal.
This Publication also discloses the technology in which by stopping the fuel injection to one particular cylinder in the group injection/ignition mode, that cylinder is made to misfire intentionally, and a cylinder discrimination is made by determining whether or not the misfire is detected. Further, this Publication discloses a method in which the mode is switched into an independent injection/ignition mode in which after the cylinder discrimination is completed, fuel is injected independently to each cylinder for each crank angle of 720.degree. according to the cylinder discrimination result.
With the method disclosed in this Publication, however, in order to make one particular cylinder misfire, the stopping of fuel injection to that cylinder must be repeated for each 360.degree. CA (crank angle) throughout plural cycles based on the above-mentioned control signal. Moreover, the cylinder discrimination cannot be made until the particular cylinder is thus made to misfire and the misfire is detected. In addition, in order to increase the reliability of cylinder discrimination, the aforesaid interruption of fuel injection and detection of misfire caused by this must be repeated. Therefore, the misfire state continues for a relatively long period of time, which is undesirable for an internal combustion engine.
Also, with this conventional method, if a mistaken cylinder discrimination occurs at the engine start, the fuel injection control is carried out according to the mistaken cylinder determination result, and this state continues, so that a problem of deteriorated fuel consumption etc. arises. Further, the fuel injection is stopped compulsorily to cause misfire, by which rotational variation is produced. Therefore, there is a possibility of a problem arising in that the internal combustion engine stops when cylinder discrimination is made (engine stop).
The present invention was made in view of the above situation, and accordingly a first object thereof is to efficiently produce cylinder discrimination at the start of an internal combustion engine. Also, a second object is to produce cylinder discrimination reliably and accurately even at a time other than at engine start. Further, a third object is to improve the reliability of cylinder discrimination, and a fourth object is to prevent a problem such as engine stopping from occurring when cylinder discrimination is produced.
Still further, a fifth object of the present invention is to enable cylinder discrimination even when the internal combustion engine is running in a steady state, and a sixth object thereof is to prevent variations in the output of internal combustion engine when cylinder discrimination is produced.
The present invention provides a cylinder discriminating scheme for an internal combustion engine which can achieve the above mentioned objects.